1. Field of the Invention
This invention relates in general to a driving circuit of an active matrix organic light emitting diode (AMOLED) pixel, and more particularly, to a driving circuit of a current-driven active matrix organic light emitting diode pixel and a driving method thereof.
2. Description of Related Art
As information technology develops continuously, new models of various information devices, such as computers, mobile phones, personal digital assistants (PDA) and digital cameras, keep being produced. Among these information devices, a display always plays a very important part, and flat panel displays are getting more popular then ever because of their thin, light, compact and power saving characteristics.
Among the variety of flat panel displays, an AMOLED display is very suitable for devices with a small size display, such as an electronic clock, a mobile phone, a PDA, or a digital camera, because of its wide view angle, good color contrast effect, fast response time, and low cost, etc.
FIG. 1 shows schematically a pixel of a conventional voltage-driven AMOLED. In FIG. 1, the AMOLED pixel comprises a switching thin film transistor 110, a driving thin film transistor 120, a capacitor 130, and an OLED 140. A gray scale to be displayed is determined by a voltage on a data line. When a voltage on the scan line is applied to a gate of the switching thin film transistor 110 (i.e., the pixel is scanned), the switching thin film transistor 110 is thus turned on, so that the voltage on the data line is transmitted to a gate of the driving thin film transistor 120. The gate voltage Vg of the driving thin film transistor 120 drives a current to flow through the OLED 140 to display. However, threshold voltages and mobilities for driving thin film transistors 120 of different pixels are different from each other since the manufacturing process is not uniform. As a result, even though the same gray scale voltage is provided, the currents flown through the OLEDs 140 will be different, causing a displayed image or screen to be not uniform.
FIG. 2 shows schematically a pixel of a conventional current-driven AMOLED. In FIG. 2, the AMOLED pixel comprises a first switch 210, a second switch 220, a third switch 230, an OLED 240, a driving thin film transistor 250 and a capacitor 260. In operation, the second switch 220 and the third switch 230 are first turned on, so that a current provided by a current source flows through the driving thin film transistor 250 to charge the capacitor 260. At this time, a gate voltage is stored in the capacitor 260. Then, the second switch 220 and the third switch 230 are turned off and the first switch 210 is turned on, so as to control the AMOLED pixel to illuminate.
The gray scale of the current-driven AMOLED pixel is determined by a magnitude of the current provided by the current source, and therefore, the gray scale will not be affected by the threshold voltages and the mobilities of the driving thin film transistors 250 of different pixels to cause an unevenness of the displayed image or screen. However, when the current-driven AMOLED prepares to display a low gray scale, because the current of the current source is small, the pixels are easily affected by parasitic resistors of the display panel and a delay effect caused by capacitors, so that the gate capacitor in the pixel cannot be charged within a predetermined scanning time. Therefore, a wrong gate voltage is stored to cause an insufficient brightness when the pixel is driven to illuminate.